1. Field
The present disclosure relates to a wax emulsion comprised of polyethylene wax. The disclosure also relates to a method of making the wax emulsion, and to emulsion aggregation toners including the wax therein having a reduction of coarse particles.
2. Description of Related Art
In certain reprographic technologies, such as xerographic and ionographic devices, smaller sized toners having, for example, a volume average particle diameter of from about 2 to about 11 microns and preferably less than about 7 microns, are desired. For example, in xerographic systems wherein process color is utilized, such as pictorial color applications, small particle size colored toners, preferably of from about 3 to about 9 microns, are desired to avoid or minimize paper curling. Also, it is preferable to select small toner particle sizes, such as from about 1 to about 7 microns, and with higher colorant loading, such as from about 5 to about 12 percent by weight of toner, such that the mass of toner layers deposited onto paper is reduced to obtain the same quality of image and resulting in a thinner plastic toner layer on paper after fusing, thereby minimizing or avoiding paper curling.
One advantageous method for preparing such smaller size toner particles is known as the emulsion/aggregation/coalescing procedure. In this procedure, the toner is achieved via aggregation as opposed to mechanical particle size reduction. Emulsion/aggregation/coalescing processes for the preparation of toners are illustrated in a number of Xerox patents, the disclosures of which are totally incorporated herein by reference, such as U.S. Pat. No. 5,290,654, U.S. Pat. No. 5,278,020, U.S. Pat. No. 5,308,734, U.S. Pat. No. 5,370,963, U.S. Pat. No. 5,344,738, U.S. Pat. No. 5,403,693, U.S. Pat. No. 5,418,108, U.S. Pat. No. 5,364,729, and U.S. Pat. No. 5,346,797. Also of interest may be U.S. Pat. Nos. 5,348,832, 5,405,728, 5,366,841, 5,496,676, 5,527,658, 5,585,215, 5,650,255, 5,650,256 and 5,501,935. In these methods, the toners are formed chemically in situ and do not require known pulverization and/or classification methods.
U.S. Pat. No. 6,582,873, incorporated herein by reference in its entirety, describes an emulsion aggregation toner made by a process in which aggregation is primarily controlled by utilizing a coagulant of polyaluminum chloride (PAC). A latex comprised of submicron resin particles of about 0.1 to about 0.4 micron in volume average diameter suspended in an aqueous phase of water, nonionic and anionic surfactants, a colorant dispersion comprising submicron colorant particles of about 0.08 to about 0.3 micron in volume average diameter, and optionally a wax dispersion comprised of submicron wax particles of about 0.1 to about 0.3 micron in volume average diameter, are added together and suspended in an aqueous phase of water and an anionic surfactant. The resultant blend is optionally stirred and heated to a temperature below the resin Tg, resulting in aggregates, the pH of the mixture is adjusted with a base, and the mixture is heated to a temperature above the resin Tg, followed by lowering the pH of the mixture with an acid to fuse the aggregates.
Waxes are added to toner formulations in order to aid toner release from the fuser roll, particularly in low oil or oil-less fuser designs. For emulsion aggregation toners, for example styrene-acrylate emulsion aggregation toners, linear polyethylene waxes such as POLYWAX® 725 (available from Baker Petrolite), are useful. To incorporate the wax into the toner, it is necessary for the wax to be in the form of an aqueous emulsion or dispersion of solid wax in water, where the solid wax particle size is usually in the range of from about 100 to about 500 nm.
Toners may contain and/or require a high amount of wax loading. For example, a styrene-acrylate emulsion aggregation toner may include 8 to 15% by weight pigment and 8 to 15% by weight wax. The combined high loading of pigment and wax, however, results in a problem that large disk shaped protrusions form at the toner particle surface during the cooling of the toner particle after coalescence. This disadvantageously can lead to formation of coarse toner particles and can cause development issues with respect to the distribution of surface additives on the toner surface.
Various solutions to this problem have been attempted, including utilization of higher melting waxes such as POLYWAX® 850, but this negatively impacts the toner minimum fixing temperature. Also attempted have been process changes such as cooling the toner particles at higher pH, but this results in formation of a rougher and less spherical toner particle.
U.S. Pat. No. 5,688,625, incorporated herein by reference, describes a process for minimizing the amount of wax that escapes from a toner, which comprises melt mixing toner resin and pigment, and injecting a water emulsified wax composition therein, wherein the generated wax domain size range is from about 50 to about 1,500 nanometers.
Although such procedures may reduce the occurrence of surface wax protrusions, a penalty is typically paid in another area such as in poorer toner performance or reduced process latitude. What is desired, then, is a procedure to readily and reliably reduce the occurrence of toner particle surface wax protrusions without compromising toner performance or limiting performance latitude.